1. Field of Use
These teachings relate generally to control systems and more particularly to the intelligent use of software deadband control in a control system to remove unwanted limit-cycling in a closed loop control system when the system is in a static position mode.
2. Description of Prior Art (Background)
A power transmission mechanism is often provided between a driving source and a driven member. Especially when position control is performed on a driven member (a load), which has relatively large inertia such as multi-axis positioning equipment. A power transmission mechanism such as a gear train connects a motor serving as a driving source to a load in many cases in consideration of the efficiency, arrangement and the like of the motor. This is often the case with a DC motor used as the driving source since high efficiency is achieved in driving at a high speed.
The power transmission mechanism generally involves a so-called mechanical dead band (hereinafter referred to as “play”) such as backlash and rattle in a gear train. When a position detector, such as a rotary encoder is directly connected to the load, this is generally referred to as a closed control system. A control system is likely to operate in an unsteady state due to the play in the gear train or the like. Also, the encoder may need to operate at a high frequency to provide a required resolution, thereby causing a higher cost to manufacture and operate. To avoid these situations, a position detector is often connected to the motor shaft.
Many known constructions of positioners, such as elevation-over-azimuth, two-axis positioners use small, high-speed electric motors with gear-trains to drive the low-speed output axes. Gear-trains have a major disadvantage-backlash, or deadband control. Excessive backlash can cause problems with stability in a closed-loop control system, and backlash adds to position error.
An example of where the deadband control problem is accentuated is in elevation-over-azimuth, two-axis positioners in which the position of the elevation axis is determined by the position of the elevation drive in the base relative to the position of the azimuth drive in the base have a major disadvantage. This is that any backlash in the azimuth drive would add to the backlash in the elevation drive, increasing the total backlash at the elevation output axis.
Direct drive electric motors have been used to eliminate backlash. They generally require more size, mass, and input power than a small, high-speed motor with a gear train for a given output power with a slow-moving output axis.
Various techniques and stabilizaton control methods are known in the art. For example, compensation filters, such as Proportional-Integral-Derivative control (hereinafter referred to as “PID control” and, Proportional, Integral, and Derivative are abbreviated as “P.” “I,” and “D,” respectively) are often used due to readiness of design and adjustment. Other compensation filters could include: lead, lag, lead/lag, and other types of suitable filters.
As described above, the power transmission mechanism has play therein. For example, when the power transmission mechanism is used to drive a member which has a relatively large inertia the play is increased. In consideration of a deviation caused by the play, it is desirable that the power from the driving source to the driven member are not separated due to play in the power transmission mechanism, or otherwise disengaged from each other immediately before the driving source stops in order to enhance the accuracy at the stop position of the driven member.
When such a driving system is subjected to position control through the, for example, PID control, in the semi-closed system, vibrations may easily occur if a high integral gain is used to seek quick elimination of the deviation of the actual driving position of the motor from the target position obtained from a speed table or position sensor.
Many methods have been used for control of backlash in gear trains. George W. Michalec's book Precision Gearing: Theory and Practice, published in 1966, has a good description of many different methods. Most have disadvantages such as increased size, weight, and cost. Examples are split, spring-loaded scissor gears and the use of auxiliary gear trains. However, spring-loaded methods tend to reduce gear life by causing excess wear and material fatigue. Typically, the higher the spring load the shorter the gear life.
Another typical dead band control practice known in the art is to form an error term, which is the point command minus the feedback position data. When the error term gets below a threshold value, the term is set to zero. In a type I or type II closed loop system with backlash, setting the term to zero prevents the system from moving back-and-forth, i.e., jittering, in the backlash zone of the gearing system, which is a form of limit-cycling. Setting the error term to zero prevents limit cycling. This lessens the wear and tear on the servo-controlled system.
However, when tracking a moving, i.e., changing or non-static, point command, the servo-controlled gearing is “pushing” to one side of the backlash zone, against the gear face, while at the same time, the servo controller is generating control signals attempting to reduce the error term to zero, or at least below the threshold value. It will be appreciated that setting the threshold to zero when the threshold is passed while the point command is changing and the gear train is dynamic will induce jitter that is a mechanical disadvantage. It will be appreciated that jitter can cause excess wear and tear on the gearing system and increase the amount of tracking error. It will also be appreciated that to compensate for these mechanically induced errors the tolerances associated with manufactured parts be as small as possible. Achieving such tolerances are difficult and expensive to maintain.